The present invention generally relates to an apparatus for electronically sensing an average temperature. More specifically, the present invention relates to a temperature sensor which uses multiple, spaced sensors such as thermistors in a circuit, such as for use in monitoring an average air temperature across an area within a large ventilation (HVAC) system.
Controlled, forced-air ventilation systems are known which move air within buildings. In many ventilation systems, fans draw fresh outside air into a building, and exhaust stale interior air to the outside. The ventilation systems use with venting or ducts to provide an air flow path throughout the building, including to and from heaters and/or air conditioners. Often the ventilations systems perform heat transfer (recovery) between the interior air to be exhausted and the outside air being introduced. For proper control of these ventilation systems, parameters such as fan speeds or damper positions are set and changed based upon sensed air temperatures within the building or within the system. Particularly in systems where air of different temperatures mixes, it is important to be able to accurately determine average air temperature, such as the average air temperature across a vertical cross-section at a location within a duct.
An early type of structure for sensing average temperature in an air duct is a fluid-filled tube or capillary. The fluid placed in the capillary, such as an oil, has a known coefficient of thermal expansion. The capillary is positioned to extend its length across the area to be temperature-averaged. As the air temperature within the air duct increases, the oil within the capillary is heated, the volume of oil expands, and the length of capillary filled by oil increases. The height of the oil within the capillary is representative of the average oil temperature along the length of the capillary. Switches or set point controls are turned (on or off) based upon the height of the oil column, allowing control of the HVAC system.
The oil-filled capillary averaging sensor had several drawbacks. First, the oil-filled capillaries were expensive. The switches or set point controls required to produce an electrical output added further expense. The capillary system had a relatively high thermal mass, and a correspondingly slow response time to changes in temperature. To minimize thermal mass, the capillaries could be made thinner, but thinner capillaries become increasingly fragile.
Today, electronically-based average temperature sensors have largely replaced the oil-filled capillary. Some electronically-based temperature sensors use a thin platinum wire to replace the oil-filled capillary. For example, platinum resistive temperature detectors were formed of a five mil thick strand of platinum, which is extended across the area to be sensed. The electrical resistance of the platinum strand changes as a function of temperature, and sensing the resistance of the platinum strand allows a determination of average temperature across the length of the strand. The platinum resistive temperature detector thus allows for direct electrical sensing of changes in temperature. Similar to the very thin capillaries, the thin platinum strands are quite fragile. To prevent damage to the very thin platinum detectors, the wire has been placed inside an electrically insulating sheath, which in turn has been placed within a flexible metal tube. The flexible metal tube can be bent and configured to fit within the ventilation channel to support the platinum strand at the positions for which temperature should be averaged.
Unfortunately, platinum is an expensive material, and even a thin strand of platinum adds significant cost. A reduced cost version of the platinum strand temperature sensor involves the use of thermisters. Thermisters, typically sintered metal oxides or alloys, also have an electrical resistance which changes based upon temperature. Thermisters can be commercially purchased at a much lower cost than the platinum strand. By treating the thermistor as a load resistor and measuring the voltage drop across the thermistor, an electrical circuit can detect changes in resistance. The sensed temperature can be obtained from an established lookup table, which provides a temperature value corresponding to the measured load resistance.
The sensing element of common thermisters is a bead about {fraction (1/10)}th of an inch in diameter. Leads extend to and from the sensing bead. To obtain an approximation of average temperature, multiple thermisters can be spaced in a circuit linking the thermisters in parallel or in series. The electrical resistance of the circuit is then indicative of the temperature at all of the thermister locations.
With the advent of electronic, computer-based controllers, lookup tables for rated thermisters have been installed in the controllers to translate between the sensed resistance and the temperature. If a parallel/series square array (2×2, 3×3, etc.) of thermisters is used, with the same number of thermisters in each series connection as number of series in the array, then the parallel/series square array has a resistance that approximates the resistance of a single thermister. Multiple point, parallel/series square thermister array temperature sensor have thus replaced the platinum strands and become commonly used for approximating average temperatures. The entire electrical circuit of the multiple point, parallel/series square thermister array has been placed in the metal tube of the prior platinum strand art for protection and support of the circuit.
Assembly of the electrical circuit of thermister arrays has been problematic. An insulative card has been used, allowing solder points between the leads for the thermister to the wires extending between thermister locations. The metallic nature of the tube requires dielectric insulation to prevent electrical shorting between the thermistors and the wall of the tubing. The insulation/metal tube support and protection configuration thermally insulates the platinum strand from the air, slowing the response time of the averaging temperature sensor. The soldering card further adds thermal ballast to slow response time. Response time in the control systems is fairly significant, because delays in control can lead to damage to system elements, particularly if the system manipulates outside air at a drastically different temperature than the inside air.
When the parallel/series square thermister array is housed in a tube, the actual placement of the thermisters within the tube is difficult to ascertain. Bending the tube improperly can cause inadvertent crimping and/or kinking of the metal, which could effectively sever the electrical connections or which could lead to small holes forming in the tube. Where small holes in a metal tube are created, cycled temperature differences can result in condensation on the inside of the tube which sometimes affect the accuracy of the temperature sensor. Condensation at the location of a thermistor could short-circuit the thermistor and lead to anomalous temperature readings.
The solder connections are exposed to tension and stresses associated with adjusting and bending the wires. Over time, the solder points weaken and electrical connections break. The resulting open circuit may be difficult to locate if the wire is placed inside a tube, and may be costly to repair no matter how the sensor is situated.